CN113711398B - Systems and reactors for electrical energy storage - Google Patents

Abstract

The present invention relates to energy storage systems and reactors useful in such systems. The reactor of the invention comprises a reaction vessel and a compensating element defining an inner volume, whereby said inner volume is filled with a fixed bed which is free of cavities and comprises particles of formula (I), feOx (I), wherein 0.ltoreq.x.ltoreq.1.5; the compensation element is adapted to adjust the inner volume. The reactor itself is explosion-proof and thus suitable for domestic use. The system can be used to compensate for long term fluctuations observed in renewable energy production.

Description

Systems and reactors for electrical energy storage

Overview

The present invention relates to energy storage systems and reactors useful in such systems. The reactor itself is explosion proof and therefore suitable for domestic use.

background

It is generally accepted that energy generation and consumption fluctuate; therefore, the grid needs to compensate for short-, medium- and long-term fluctuations. Renewable power sources (e.g. photovoltaics, wind power) depend heavily on weather conditions. Therefore, the production of renewable energy fluctuates on multiple time scales: hourly, daily and seasonal. In order to always maximize renewable energy production, the temporary surplus between the energy produced and the energy consumed must be stored for a period of time.

For example, the energy generated by a photovoltaic system is used to meet the electrical energy needs of one or more households. If more electrical energy is produced than consumed, the electrical energy is stored by charging the battery. Conversely, if less electrical energy is produced than consumed, the missing energy is provided by discharging the battery. If the battery is fully charged, for example in the summer, the surplus energy is supplied to the grid, usually with no or only little benefit. If the battery is completely discharged, for example in the winter, the missing energy is purchased from the grid at a relatively high price. Instead of buying and selling energy at unfavorable prices, it is more beneficial to store the surplus energy produced throughout the season and release it in another season of the year. Therefore, the capacity of the seasonal energy storage system needs to be significantly larger than that of the daily energy storage system. In this context, it is important to note that, in addition to the high cost, rechargeable battery systems generally tend to discharge slowly if the storage exceeds half a year, such as is required for seasonal energy storage.

Energy storage systems using the redox pair Fe ₃ O ₄ /Fe are known and described in DE 10 2017 2018 61. The system disclosed in this document is complex and requires the solid material (Fe/Fe ₃ O ₄ ) to be moved from the storage device to the reactor and the solid-gas separator. Such systems require a high level of maintenance and/or are prone to failures and are therefore not very suitable for home use. In addition, the system contains hydrogen and carries its own risk of explosion accidents. Such risks require additional measures and are therefore generally unacceptable to customers, especially in home applications.

Redox pairs of iron oxide/iron and their use for energy storage are known and well studied. For example, Selan et al. (J. of Power Sources 61, 1995, 247) examined a process cycle of sponge iron/hydrogen/iron oxide. According to the authors, this cycle provides a simple possibility of storing synthesis gas energy in the form of sponge iron. In addition, Pineau et al. (Thermochimica acta, 456, 2007, 75) studied the kinetics of hydrogen reduction of iron oxide. Neither of these two documents discloses any specific teachings on energy storage systems.

Therefore, there is a need for further, in particular improved, energy storage systems.

It is therefore an object of the present invention to alleviate at least some of these disadvantages of the prior art.In particular, it is an object of the present invention to provide an energy storage system that is more reliable and/or safer than known systems.

These objects are achieved by a system as defined in embodiment 6 and a reactor suitable for use in such a system as defined in embodiment 1. The description and the independent claims disclose further aspects of the invention, and the description and the dependent claims disclose preferred embodiments.

Summary of the invention

The present invention will be described in more detail below. It should be understood that the various embodiments, preferences and ranges provided/disclosed in this specification may be combined at will. In addition, depending on the specific embodiment, the selected definition, embodiment or range may not be applicable.

Unless otherwise stated, the following definitions apply to this specification:

As used herein, the terms "a", "an", "the" and similar terms used in the context of the present invention (especially in the context of the claims) should be interpreted to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by the context. As used herein, the term "containing" should include "comprising", "consisting essentially of", and "consisting of".

The term "fixed bed", also referred to as "packed bed", is known in the art. It refers in particular to a fixed and immovable packing material located within a reactor vessel. A fixed bed is not agitated by a high-quality fluid stream or a stirring device and is therefore different from a fluidized bed or a moving bed. The packing material of a fixed bed is suitable for allowing one or more fluids to pass through the bed, thereby allowing a chemical reaction. Suitable packing materials include granular (e.g., powders, particles, etc.) products (e.g., Raschig rings) and structured packings. A fixed bed can be characterized by inter-particle porosity, intra-particle porosity, and total porosity, see Figures 4A to 4D. Porosity can be determined by standard methods, in particular the standard methods described below.

The term "particle" is known in the art. It refers in particular to a material in a specific form that remains together under a given mechanical treatment. Particles can be characterized by their appearance (including porous, non-porous, granular), particle size, particle size distribution, porosity, density and specific surface area. Particles can be solid, which means that they are completely composed of solids and have a well-defined, continuous outer surface without openings or pores. The entire particle volume is filled with material. Typical examples of solid particles are particles formed by the solidification of droplets of molten materials such as glass, wax or metal. The surface tension of these materials attempts to produce particles with a minimum surface and close openings and pores during the manufacturing process. Therefore, the particle density (ρ_particle) of a solid particle is equal to the material density (ρ_material) of the solid material constituting the particle.

For solid particles, the particle volume can be obtained from the particle mass and the material density. The density of typical materials is a tabular property and depends on the chemical composition of the material.

Particles can also be porous. The volume of a porous particle is filled with solid material and void space. Porous particles can have at least one closed void in their interior (inaccessible vacuum within a solid), or they have at least one opening or pore on their outer surface. Porous particles usually come from a drying process or mechanical treatment, where the components of the porous particle are brought together and then formed into a granule (particle).

In order to distinguish the open spaces between particles and within particles (the portion of space not filled with solids), the concepts of inter-particle porosity and intra-particle porosity are used.

Interparticle porosity (= void fraction, bed porosity), ε_bed: the space between solid particles, expressed as a percentage or fraction of the total volume. The interparticle porosity of a fixed bed is used to indicate the space available for fluid flow to carry out chemical reactions. The interparticle porosity of a fixed bed is the ratio of the volume of the voids between the particles to the total volume of the bed. This value depends on the shape and size distribution of the particles, the ratio of the particle size to the bed diameter, and the method used to pack the bed. The volume of the voids is the volume of the bed minus the volume of the particles. For spheres of uniform size, the porosity range that can be achieved with ordered packing is a value of 0.26 (diamond packing) to 0.48 (cubic packing). Disordered or random packing of spheres typically ranges from 0.44 (very loose random packing) to 0.36 (tight random packing).

The volume of a particle (V_particle) can be calculated using the particle size and shape, which can be determined using optical methods such as optical microscopy or electron microscopy. For spherical particles, the calculation of the volume is simple (V_particle = 4/3·pi·r ³ ; r is the radius of the particle, and pi is 3.14). For irregularly shaped particles, approximate methods are often used, the most common of which is to use the average diameter of multiple measurements to obtain the average radius (r = d/2).

For porous particles, the volume of the particle includes possible voids or pores inside the particle. For clarity, the mass of a porous particle should be less than the particle volume multiplied by the material density (ρ_material) because the density of voids or pores is usually much lower.

For particles used as pigments, DIN standard EN ISP 8130-13 describes the use of laser scattering for determining particle size. It generally provides an average size that can be used to calculate the average particle volume. This method is favorable for micron-sized materials (particle size in the range of 1-50 microns). For larger particles (over 50 microns), sieve analysis is generally used.

The interparticle porosity can be determined using the bed density (ρ_bed = m_bed/V_bed), the particle density (ρ_particle = m_particle/V_particle), and the density of the fluid filling the voids (ρ_fluid):

ε_bed = (ρ_particle-ρ_bed)/(ρ_particle-ρ_fluid)

Intra-particle porosity (= porosity), ε_particle: the fraction of the volume of a porous particle that is not occupied by the solid itself. It is usually given as a fraction, percentage or decimal. Gravel usually has a low or negligible void fraction, while sawdust or dry soil have many holes or voids. Intra-particle porosity can be determined using the particle density (ρ_particle = m_particle/V_particle), the material density (ρ_material) and the density of the fluid that fills the voids (ρ_fluid):

ε_particle = (ρ_material - ρ_part) / (ρ_material - ρ_fluid)

Total porosity ε_total: The fraction of the volume of the fixed bed that is not occupied by solid, dense material. The total porosity can be determined using the bed density (ρ_bed = m_bed/V_bed), the material density (ρ_material), and the density of the fluid that fills the voids (ρ_fluid):

ε_total = (ρ_material - ρ_bed) / (ρ_material - ρ_fluid)

In theory, the total porosity is a combination of the intra-particle porosity and the inter-particle porosity, and these porosities are related as follows:

ε_total = (V_bed·ε_bed+V_bed·(1-ε_bed)·ε_particle)/V_bed

=ε_bed+ε_particle-ε_bed·ε_particle

There are a variety of experimental techniques for determining the properties of porous materials. Gas adsorption with nitrogen is used for measurements involving pore sizes in the range of 0.4–300 nm (Klobes et al., 2006). Liquid intrusion by non-wetting liquids (i.e., mercury porosimetry) is widely accepted for scales above 4 nm, particularly between 4 nm and 60 μm. (Espinal, 2012).

In the context of the present invention, suitable particles are in particular those having a diameter of 0.01 μm to 1.0 μm (as defined in the abovementioned standards) and/or a surface area of 2 to 80 m ² /g.

The present invention will be better understood with reference to the accompanying drawings , in which Figures 1A, 2A and 3A illustrate the charging mode of the system of the present invention, while Figures 1B, 2B and 3B illustrate the discharging mode of the system of the present invention.

Reference numerals list

1Reversible fuel cell

1a Water inlet,

1b Hydrogen outlet,

1c water/oxygen outlet,

1d Gas inlet,

1e Hydrogen inlet

1f Water/gas outlet

2 Reactor

2a Reactor inlet; 2b Reactor outlet

3 separation units, separation of water and hydrogen

3a Separator inlet,

3b Hydrogen-rich phase outlet,

3c Water-rich phase outlet

8 Electrolyzer

8a Water is like a mouthful,

8b Hydrogen outlet,

8c Water/oxygen outlet

9. Fuel Cells

9a Gas inlet,

9b Hydrogen inlet,

9c water/gas outlet,

9d Hydrogen export

10 The first separation unit separates water and hydrogen for charging

10a Separator inlet,

10b hydrogen-rich phase outlet,

10c Water-rich phase outlet

11 The second separation unit separates water and hydrogen for discharge

11a Separator inlet,

11b hydrogen-rich phase outlet,

11c Water-rich phase outlet

4 Hydrogen cycle

5 Water cycle

6 Hydrogen purification

7 Water supply

12a power input (charging), 12b power output (discharging)

13Energy Storage System

Figures 1A and 1B show a first embodiment of the system of the present invention, comprising a reactor (2), a reversible fuel cell (1) and a separation unit (3) as described herein. In the charging mode according to Figure 1A, the power input (12a) is in operation and allows the reduction reaction in the reactor (2) as described herein. In the discharge mode according to Figure 1B, the power output (12b) is driven by the oxidation reaction in the reactor (2) as described herein.

Figures 2A and 2B show a second embodiment of the system of the invention, comprising a reactor (2) and a separation unit (3) as described herein. In this second embodiment, the reversible fuel cell is replaced by an electrolyzer (8) and a fuel cell (9) compared to the first embodiment. In the charging mode according to Figure 2A, the power input (12a) is in operation and allows the reduction reaction in the reactor (2) as described herein; the fuel cell (9) is not in operation. In the discharge mode according to Figure 2B, the power output (12b) is driven by the oxidation reaction in the reactor (2) and the fuel cell (9) and operates. The electrolyzer (8) is not in operation.

Figures 3A and 3B show a third embodiment of the system of the present invention, comprising a reactor (2), an electrolyzer (8) and a fuel cell (9) as described herein. In this third embodiment, compared to the second embodiment, the separation unit (3) is replaced by a first separation unit (10) for charging and a second separation unit (11) for discharging. In the charging mode according to Figure 3A, the power input (12a) is in operation and allows the reduction reaction in the reactor (2) as described herein; the fuel cell (9) and the second separation unit (11) are not in operation. In the discharge mode according to Figure 3B, the power output (12b) is driven by the oxidation reaction in the reactor (2) and the fuel cell (9) and operates. The electrolyzer (8) and the first separation unit (10) are not in operation.

4A to 4D show solid particles (FIG. 4A), inter-particle volume of solid particles (FIG. 4B, hatched), porous particles (FIG. 4C), intra-particle volume of porous particles (different hatched), and inter-particle volume of porous particles (FIG. 4D).

Figures 5A to 5C show how to assess the size of a cavity. Figure 5A shows a general ellipsoid and its three principal radii, a, b, and c. Figure 5B shows a solid material and a cavity. Figure 5C shows a solid material, a cavity, and the largest ellipsoid that fits the cavity.

More generally, in a first aspect , the present invention relates to a specific reactor 2 comprising a reaction vessel 2c defining an inner volume, a compensating element 2d, an inlet 2a and an outlet 2b, a heating element 2e and an insulator 2f, characterised in that the inner volume is filled with a fixed bed and the compensating element is suitable for regulating the inner volume. The fixed bed (i) does not have large cavities as further defined in detail below. In addition, as further defined in detail below, the fixed bed comprises or consists of particles of formula (I), i.e. FeOx (I), wherein 0≤x≤1.5. The reactor 2 of the present invention allows the conversion of hydrogen into water (thereby capturing the chemical energy obtained thereby) and its reverse reaction (thereby releasing hydrogen). The reactor itself is explosion-proof and is therefore suitable for domestic applications. This aspect of the invention will be described in further detail below:

Reactor 2: The reactor is a unit capable of carrying out chemical reactions, in particular the reduction and oxidation of FeOx(I) as discussed herein. The reactor of the present invention comprises a number of elements, in particular a reaction vessel, one or more compensating elements, one or more inlets and outlets, one or more heating elements, an insulator. Other elements may be present. The technician is familiar with equipping the reactor with all the elements required to carry out the reactions described herein and to integrate the reactor into the system described herein (second aspect). The technician is also familiar with selecting materials suitable for the reaction conditions described herein. Requirements for the reactor material include long-term stability at high temperatures during exposure to hydrogen and/or steam. Suitable materials for the reactor vessel include austenitic stainless steels with enhanced corrosion resistance and low potential for hydrogen damage, such as 316L (1.4404) type and 316Ti (1.4571) type. Suitable sealing materials include expanded graphite sheets.

As described below, the reactor operates at ambient pressure and a temperature in the range of 500° C. Thus, reactor 2, in particular reaction vessel 2c, is suitable for pressures below 2 bar and temperatures below 1000° C.

Reaction vessel 2c: The internal volume of the reaction vessel can vary within a wide range, with a suitable internal volume being 1-10 m ³ .

The shape of the reaction vessel may generally be any shape known in the art; its design particularly takes into account insulation and heating as well as manufacturing. Suitable are reaction vessels in the form of cylinders or spheres or tube bundles, with cylindrical form being preferred. Preferably the external geometry of the reactor vessel is simple, e.g. cylindrical or cubic, and has a low surface area to volume ratio to minimize heat losses.

As described herein, the reaction vessel is filled with particles of formula (I) in the form of a fixed bed. Therefore, it is not necessary to include means for agitation and/or transportation. This is considered to be particularly advantageous because it makes the reactor more stable and is particularly useful in domestic applications.

In one embodiment, the fixed bed consists of particles of formula (I) in the form of a compacted powder, such as pellets. The compacted powder may be shaped in a manner that enhances convective transfer of mass and/or heat within the fixed bed.

In one embodiment, the fixed bed is configured in a manner to enhance convective transfer of mass and/or heat within the fixed bed.

Compensating element 2d: comprises one or more compensating elements. This term describes an element that compensates for changes in the volume of the fixed bed (compression/expansion=breathing) during the process. The compensating element is thus able to adjust the internal volume. During the reaction described below, the particles of formula (I) are oxidized or reduced. As a result, the particle size changes. In order to ensure that the internal volume is free of cavities, a compensating element is implemented. The compensating element is thus suitable for compensating for volume expansion/compression during operation of the reactor. Such compensating elements are known per se; they can be formulated in the form of a membrane or in the form of a piston or in the form of a sponge or in the form of a fibrous material. Suitable fibrous materials are known and include inorganic materials obtained by rotating or stretching molten mineral or rock materials. Examples of such fibrous materials include mineral wool, such as commercially available rock wool, slag wool and glass wool.

Inlet, outlet 2a, 2b: The inlet and outlet are elements suitable for filling the internal volume and removing gas from the internal volume. In addition, they are suitable for retaining particulate material in the reactor vessel. Commercial valves can be used. Obviously, such inlet and outlet are in fluid communication with other units of the system described below. Depending on the operating mode, the functions of the inlet and outlet can be interchanged, i.e. inlet in charging mode and outlet in discharging mode. Typically, the inlet and outlet are located on opposite sides of the reactor, for example at the top and bottom of the reactor 2.

Heating elements 2e: The heating of the fixed bed is to achieve and maintain the specified temperature for the charging and discharging process. Internal heaters can be used; they provide thermal energy directly to the fixed bed. Alternatively or additionally, external heaters can be used; they provide energy to the reactor vessel or to the fluid entering the reactor.

The temperature of the charging reaction and the temperature of the discharging reaction are affected by multiple criteria. Lower temperatures are favorable for heat loss in the system and aging of the oxygen carrier material, while the reaction kinetics are enhanced at higher temperatures. In terms of thermodynamic equilibrium between the reaction atmosphere and the solid oxygen carrier material, higher temperatures are favorable during charging, while lower temperatures are favorable during discharging. Suitable ranges include:

Charging: 200°C-450°C, preferably 200°C-400°C, most preferably 200°C-350°C.

Discharge: 150°C–400°C, preferably 150°C-350°C, most preferably 150°C-300°C.

Insulator 2f: The reactor 2 comprises a thermal insulator; such insulators are known per se. Thermal insulation of the reactor vessel reduces the amount of thermal energy that must be supplied to the reactor. The requirements for the insulator are low thermal conductivity and long-term stability at the corresponding temperatures. Suitable insulators include rock wool and/or a vacuum chamber.

Other Elements: As mentioned above, the reactor may include one or more additional elements.

In one embodiment, a sensing element is included, such as a temperature sensor and/or a pressure sensor.

In one embodiment, a gas tank is included in fluid communication with the reaction vessel. Such a gas tank may contain an inert gas (e.g., nitrogen) to regulate gas pressure and volume during operation.

In one embodiment, the reactor is equipped with a heat exchanger. The amount of heat energy required can be reduced by heat recovery. For example, the stream entering the reactor can be heated in the heat exchanger by the stream leaving the reactor.

Particles of formula (I): Formula (I) includes iron in +/-0, +2 and +3 oxidation states and thus includes the following ideal formula:

Fe (Ia), x = 0, Fe ^{+/- 0}

Fe0＝FeO ₁ (Ib); x＝1,Fe ⁺²

Fe ₃ O ₄ =FeO _4/3 (Ic); x = 4/3, Fe ⁺² and Fe ⁺³

Fe ₂ O ₃ =FeO _3/2 (Id), x=1.5, Fe ⁺³ .

The above formula (I) also includes non-stoichiometric compounds, for example wüstite (Fe _1-z O, z<1) and mixtures of these compounds. Phase diagrams showing the stability of iron oxides at various temperatures are known.

The above formula (I) also includes hydrates of the compounds of formula (I). For example, Fe(OH) ₂ =FeO x H ₂ O is covered by formula (Ib); similarly, Fe ₂ O ₃ x nH ₂ O is covered by FeO _3/2 .

The above particles may be doped or mixed with other elements, which improve the stability of the oxygen carrier to sintering and/or promote its reduction reaction under process conditions and/or promote its oxidation reaction under process conditions. Such dopants are preferably present in an amount of 0-30%, more preferably 0-10%, most preferably 0-3%. Examples of such dopants include Ce, W, Mo, Cr, Al, Si, Ca, Mg, Ti, V, Mn, Ni, Co, Cu, Pd, Pt, Rh, Zr, Y, Gd, Zn in metallic form or in any oxidized form thereof.

Particles of formula (I) are available from many natural and synthetic sources. Such particles are commercial products and can be obtained, for example, as pigments from various suppliers.

The reactor can be loaded with iron oxide (usually preferred) as the latter is an easy to handle material (not flammable).

In some embodiments, it is preferred to load a new reactor with pre-reduced iron oxide or iron metal particles (at the time of delivery to the customer). If installed in the fall or winter, such a loaded reactor can be delivered and enable the household or consumer to begin using energy from the system directly.

No cavity filling: The particles of formula (I) substantially fill the above-mentioned internal volume of the reaction vessel 2c. Thus, the fixed bed constitutes at least 90%, preferably at least 95%, more preferably at least 98% of the said internal volume. The remaining volume can be occupied by other elements of the reactor, such as structural elements, stabilizing elements or heating elements.

As mentioned above, the fixed bed containing or consisting of particles of formula (I) has no large cavities. The term cavity refers to the space within the reactor volume that does not contain particles of formula (I). Such a cavity is defined by (i) a maximum volume and (ii) a maximum length. The volume of a cavity is the volume of the largest ellipsoid that fits into the cavity. The volume of the ellipsoid is _Vcavity = _Vellipsoid =4/3·π·a·b·c, where a, b and c are the three main radii of the ellipsoid.

In one embodiment, there is no cavity (i) where _Vellipsoid is greater than _Vmax = ¹⁰⁰⁰⁰ mm3 and (ii) all three main radii describing the volume are at least _cmin = 3 mm. In view of the above formula, the maximum extension of the cavity is below a = b = 3 mm and c = 265 mm.

In yet another embodiment, there is no cavity (i) where _Vellipsoid is greater than _Vmax = ^{2000 mm3} and (ii) all three major radii describing the volume are at least 3 mm.

Due to this cavity-free filling of the reactor, accumulation of hydrogen is prevented. The reactor itself is therefore explosion-proof. This is a significant advantage over the prior art, since no specific safety measures have to be taken. The reactor is therefore suitable for domestic use.

Other considerations and embodiments of Reactor 2 are summarized below:

In one embodiment, reaction vessel 2c is divided into two or more zones (sector), thereby defining two or more internal volumes separated from each other in space. In such an embodiment, preferably each of the zones includes a temperature sensor and a heating element. This allows uneven temperature distribution in the reactor, thereby reducing heat loss, because only a portion of the reactor wall is at high temperature. This arrangement is advantageous, because the entire reactor is maintained at high temperature to facilitate the aging (particularly sintering) of the compound of formula (I). The life of the reactor with multiple zones of independent heating is longer than the reactor in which all contents are maintained at operating temperature. Depending on size and application, it is preferred to manufacture the simplest reactor with only a few sensors and one or more heating elements, or a segmented reactor with more sensors and heating elements.

In another embodiment, reaction vessel 2c is communicated with the gas ballast fluid of inert gas.Implementing such element is a further safety measure.Use at least one element containing inert gas, typically a pressurized gas cylinder (gas cylinder) full of argon or nitrogen, which is connected to the internal volume of the reactor and controlled by at least one pressure measuring element.This element containing inert gas can be used in an emergency to balance the pressure reduction caused by cooling in the reactor (for example, if the reactor switches to release hydrogen from storing hydrogen, the latter is usually at a lower temperature), or if the reactor is closed (for example, for maintenance or control).If iron oxide is lower than stoichiometric, and the amount of steam is not equal to the amount of hydrogen absorbed, then the internal pressure reduction may also be the result of absorbing hydrogen by reacting with iron oxide.In most cases, two (one spare) nitrogen-filled gas cylinders will be used, and during system maintenance, such nitrogen can be refilled.During normal operation, the injection of inert gas is not preferred because it dilutes hydrogen and remains idle in the fuel cell.

In a second aspect, the present invention relates to an energy storage system 13 suitable for converting electrical energy into chemical energy and storing the chemical energy ("charging") and the reverse process ("discharging"). The key unit of such a system 13 is the reactor 2 described herein. The system of the present invention can be used to compensate for long-term fluctuations observed in renewable energy production. This aspect and various embodiments of the system are explained in more detail below:

In one embodiment, referring to Figures 1A and 1B, the present invention provides an energy storage system 13 comprising a reactor 2 as described herein, which is in fluid communication with a unit operating as a reversible fuel cell 1 and a separation unit 3 that separates water from hydrogen. These three units - reactor 2, reversible fuel cell 1 and separation unit 3 - are considered to be the basic elements of the energy storage system of the present invention.

In advanced embodiments of the system of the present invention, these units 1, 2 and 3 are divided into separate units (e.g., unit 1 is divided into units 8 and 9; unit 3 is divided into units 10 and 11) or the units are equipped with other elements, as further described in detail herein.

In yet another embodiment, referring to Figures 2A and 2B, the present invention provides an energy storage system 13 comprising a reactor 2 as described herein, the reactor being in fluid communication with an electrolyzer 8 and a separation unit 3 for separating water from hydrogen. The unit 3 is in turn in fluid communication with a fuel cell 9.

Obviously, according to this embodiment, the reversible fuel cell 1 is replaced by an electrolyser 8 and a fuel cell 9 .

In yet another embodiment not shown in the figures, the present invention provides an energy storage system 13 comprising a reactor 2 as described herein, which is in fluid communication with the first separation unit 10 during the charging mode and in fluid communication with the second separation unit 11 during the discharging mode.

Obviously, according to this embodiment, the separation unit 3 is replaced by a first separation unit 10 (in operation during charging) and a second separation unit 11 (in operation during discharging).

In another embodiment, referring to Figures 4A to 4D, the present invention provides an energy storage system 13 comprising the reactor 2 described herein, wherein the reactor 2 is fluidically connected to the electrolyzer 8 and the first separation unit 10 during the charging mode, and the reactor 2 is fluidically connected to the second separation unit 11 during the discharging mode, and the second separation unit 11 is in turn fluidically connected to the fuel cell 9.

Obviously, according to this embodiment, the reversible fuel cell 1 is replaced by an electrolyzer 8 and a fuel cell 9. Further, the separation unit 3 is replaced by a first separation unit 10 for a charging mode and a second separation unit 11 for a discharging mode.

In another embodiment not shown in the figure, the water-hydrogen separation unit 3 'is integrated in the reactor 2. This can be achieved by a storage element ("chamber"), which is located in the reactor and contains a microporous hygroscopic material (II). Suitable materials (II) include zeolites. Such a unit 3 'can replace units 3, 10 or 11, or be implemented in addition to any of the above units.

Except for the reactor 2, the above units are known per se and can be scaled and/or adapted by the skilled person to suit the purposes described herein. Further details about these units are provided below:

The reactor 2 is the unit of the first aspect of the present invention described above.

The separation unit 3, 10, 11 separating water from hydrogen is a unit capable of separating the two components. Such units are known and comprise a condenser and a dryer.

The conversion of hydrogen to water during charging is thermodynamically limited, i.e., the stream leaving the reactor contains both hydrogen and water. Recycling unreacted hydrogen to the reactor (4 in Figures 1A, 2A, and 3A) requires the removal of water in the separation unit (3, 10, 11 in Figures 1A, 1B, 2A, 2B, 3A, and 3B).

Established methods for separation of water and hydrogen include condensation of water, adsorption of water in a dryer and membrane separation. Depending on the detailed media available locally and their characteristics (water, heat source, size of installed PV equipment, season, geographical location), different technologies are preferred:

a) Condensing separators: In areas where cooling water is plentiful or in cold climates, condensing water and hydrogen separators are the most cost-effective solution. Where heat is readily available, absorbent-based water and hydrogen separation may be the best option. Such systems can operate over a wide temperature range, depending on the type of absorbent. For example, silica gel is available at conditions starting at 50 degrees Celsius and can operate up to over 200 degrees Celsius. Systems based on zeolites and molecular sieves typically require higher water desorption temperatures. However, this is advantageous because the water-rich hydrogen gas stream leaving the reactor is at an elevated temperature, and in an ideal case, the water/hydrogen separation occurs at a similar or identical temperature, thus operating essentially isothermally. This is highly preferred because there is essentially no need to cool and reheat the hydrogen gas stream. In some cases, membranes can be used to separate the hydrogen/water stream. Membranes are of interest when space limitations are important in the design of the particular form of the invention described herein.

b) Dryer: A dryer is a container filled with a fixed bed through which the mixture to be separated can flow. The fixed bed consists of at least one material from the group of water-absorbing materials. The group of water-absorbing materials includes zeolites (molecular sieves), silica gel and activated alumina.

The dryer may be included in the container described herein, i.e., it may occupy a portion of the reactor interior volume. Due to safety restrictions, this arrangement is of interest because all hydrogen processing is done in a well-defined, well-shielded area. In addition, heat losses due to additional walls and piping are reduced. Therefore, a dryer located inside the reactor or directly adjacent to a significant contact area of the reactor, such as a common wall, is generally preferred. In this arrangement, the valve may be located outside the reactor to reduce thermal stress on this more sensitive equipment.

The dryer operates at at least two temperatures, a first temperature (lower temperature) at which water is removed and the hydrogen system is dried, and a second temperature (higher temperature) at which water leaves the dryer and regenerates (dries) the material in the dryer. In addition to the temperature swing operation described above, other dryers are known that operate in pressure swing or pressure-temperature swing operation. The use of such dryers is generally feasible, but the temperature swing system is considered to be simpler and therefore more preferred.

Instead of a single separation unit 3, there may be a first and a second separation unit 10, 11. The first separation unit 10 removes _H2O from the stream leaving the reactor during charging. The separation may be based on a molecular sieve that absorbs water, or a membrane with a higher permeability to one of the gases, or on condensation of steam. The second separation unit 11 separates the stream leaving the reactor during discharge into a stream of a higher hydrogen fraction and a stream of a lower hydrogen fraction. The separation may be based on a molecular sieve that absorbs water (operated in a wave mode), or a membrane with a higher permeability to one of the gases, or on condensation of steam.

The reversible fuel cell 1 is a unit combining the functions of a fuel cell and an electrolyser. Such reversible fuel cells are commercial products or can be manufactured according to known methods.

The fuel cell 9 is an electrochemical generator in which the reactants are supplied from the outside. According to the invention, hydrogen is combined with air to produce electrical energy. Fuel cells are commercial products. Suitable are, for example, PEM fuel cells.

The electrolyzer 8 is a unit that decomposes water into oxygen and hydrogen, thereby consuming electrical energy. The electrolyzer is a commercial product. A suitable one is, for example, a PEM (proton exchange membrane) electrolyzer.

In addition to the above-mentioned units, system 13 of the present invention can be combined with other units, particularly in order to improve efficiency and/or safety. The fluid communication between the units is realized by a piping system, and the piping system includes a pipe, a valve and a sensor, which is conventional in the art. As mentioned above, reactants, i.e. hydrogen, water and air, pass through each unit of the system of the present invention, and the compound of formula (I) is not transported and retained in the reactor 2. This has substantial advantages over the system known from DE 102017201861.

Embedding of the System: In one embodiment, one or more heat consuming or generating elements of the system 13 of the present invention are connected to the water treatment loop of a residence. This allows the use of the water loop of the home as a cooling source, or, it allows for near instantaneous heating capabilities for the home, since the reactors described herein are large and can act as a thermal buffer for warm water that is otherwise not continuously used by the home. This near instantaneous water heating system is considered to be the best solution for storing warm water in tanks, which is widely used today, because the overall (complete) heat loss is lower if the warm water is made on site when needed.

The system is electrically connectable or electrically connectable, in particular connectable or connected to a local energy management system via power input and output 12a, 12b. The power input 12a may be an electrical connection to a photovoltaic system. The power output 12b may be a connection to an AC/DC converter. The local energy management system may be connected to an electrical grid.

Pumps, compressors: In addition, a pump or compressor can make the hydrogen flow for hydrogen recycling. The separated water can be stored for use in the discharge step. Alternatively, it can also be used in the fuel cell during the charging step. Since the components of the present invention generally require high purity water, it is generally preferred to reuse the water within the system. In addition, a pump or compressor can achieve the flow required for water recycling. The separated hydrogen can be used in the electrolyzer. Hydrogen compressors or pumps are commercially available in the form of diaphragm pumps, rotary pumps, piston pumps, etc. Since only low pressure differences are used in the system described herein, rotary pumps and diaphragm pumps are suitable. Preferably, the pump is encapsulated with an airtight shell so that it is impossible to release hydrogen.

Measurement/control system: The energy storage systems described herein generally require a control unit comprising a computer or microcontroller. In some cases, remote control options using data transmission can also be used. Although controllers are known to those skilled in the art, the systems described herein require at least one pressure controller, at least one temperature measurement in the reactor. Typically, multiple temperature measurements are used to better control the temperature distribution inside the reactor.

Safety: The system of the present invention benefits from improved safety, which will be explained in further detail: As a first safety measure, the reactor and related units/elements, such as water separators or valves and sensors, are preferably arranged in a manner that minimizes the length of the pipes, the number of connectors and the outer wall area to separate the hydrogen-containing part from the outside (air). Although the latter is a common standard in good engineering practice, the interior of the reactor is characterized in that it is essentially full, as described above. The second safety measure is that the system of the present invention operates at extremely low pressure. The reactor operates essentially under atmospheric conditions, thereby reducing the outflow of hydrogen (pressure slightly higher than the atmospheric pressure of the outside air) or the inflow of air (if the pressure of the reactor is lower than the external atmospheric pressure) in the event of a leak. In both cases, a dangerous mixture of hydrogen and air may lead to a flame or explosion. This second safety measure further contributes to the weight and manufacturing cost of the reactor because thinner walls with lower overall metal requirements can be used. The third (optional) safety measure is the use of at least one element containing an inert gas as described above and in fluid communication with the reactor internal volume. The fourth safety measure is the absence of moving elements (except for the compensation element 2d and the pump/compressor).

In a third aspect, the present invention relates to the use of a reactor and system as described herein and a method for storing/releasing hydrogen by operating a reactor 2 as described herein and storing/releasing electrical energy by operating a system 13 as described herein. This aspect of the invention will be described in further detail below:

In one embodiment, the present invention relates to a method for storing hydrogen, comprising the step of reducing a compound of formula (I) in a reactor as defined herein by supplying a _H2- containing gas to said reactor, thereby obtaining a reduced compound of formula (I) and water.

In one embodiment, the present invention relates to a method for generating hydrogen comprising the step of oxidizing a compound of formula (I) in a reactor as defined herein by feeding water to said reactor, thereby obtaining an oxidized compound of formula (I) and hydrogen.

In one embodiment, the present invention relates to a method for storing electrical energy, which comprises (a) electrolytically reducing water to obtain hydrogen; (b) reducing a compound of formula (I) in a reactor as defined herein by supplying a hydrogen-containing gas obtained in step (a) to the reactor, thereby obtaining a _H2O / _H2 gaseous mixture; (c) separating _H2 from the gaseous mixture and recycling the _H2 to the reactor.

According to the invention, electrical energy is consumed by conversion into chemical energy in an electrolyser producing hydrogen (H ₂ ). The produced hydrogen is passed through a fixed bed reactor. The fixed bed consists of iron-based oxygen carriers in oxidized form (FeOx, where 4/3≥x≥0). At elevated temperature, the oxygen carriers are reduced by hydrogen, producing steam (H ₂ O) and reduced form of the oxygen carriers (FeOy, where y<x):

(xy)H ₂ +FeO _x ->(xy)H ₂ O+FeO _y

The stream leaving the reactor consists of unreacted hydrogen and steam. This stream is fed to a first separation unit where steam is (partially) removed from the stream. The resulting stream has a higher hydrogen fraction _H2 /( _H2O + _H2 ) and is fed back to the reactor.

In one embodiment, the present invention relates to a method for releasing electrical energy, which method comprises (d) oxidizing a compound of formula (I) in a reactor as defined herein by feeding water to said reactor, thereby obtaining an oxidized compound of formula (I) and a _H2O / _H2 gaseous mixture, (e) separating _H2 from said gaseous mixture, and (f) electrochemically reducing the hydrogen obtained in step (e) to obtain water and electrical energy.

According to the invention, water (in the form of steam) is passed through a fixed bed reactor. The fixed bed consists of an iron-based oxygen carrier (I') in reduced form. At elevated temperature, the oxygen carrier is oxidized by the steam to produce hydrogen and the oxygen carrier (I'') in oxidized form:

(xy)H ₂ O+FeO _y ->(xy)H ₂ +FeO _x ; hence x>y

(I`)(I``)

The stream leaving the reactor is composed of unreacted steam and hydrogen. This stream is fed to a second separation unit where it is divided into a stream of a higher hydrogen fraction and a stream of a lower hydrogen fraction. The stream of the lower hydrogen fraction is fed back to the reactor. The stream of the higher hydrogen fraction is fed to a fuel cell that consumes hydrogen where chemical energy is converted into electrical energy. The unconverted hydrogen stream leaves the fuel cell and can be fed to a second separation unit.

In one embodiment, the present invention relates to the use of a reactor 2 as described herein for: (i) converting hydrogen into water by reducing a compound of formula (I) and storing the energy obtained thereby, and/or (ii) converting water into hydrogen by oxidizing a compound of formula (I), thereby releasing previously stored energy.

In one embodiment, the present invention relates to the use of the system 13 described herein for: (i) storing electrical energy, thereby charging the system; and/or releasing electrical energy, thereby discharging the system. Preferably, the system is charged in the summer using surplus energy from renewable energy sources, and is discharged in the winter to make up for the lower production of renewable energy sources.

The above method and use are further described in detail with reference to Figures 1A, 1B, 2A, 2B, 3A and 3B. In the charging mode (Figures 1A, 2A, 3A), electrical energy is used to generate hydrogen. The hydrogen thus generated is used to reduce the compound of formula (I). In the discharge mode (Figures 1B, 2B, 3B), water reacts with the reduced compound of formula (I) to obtain an oxidized compound of formula (I) and hydrogen. The hydrogen thus generated is electrochemically oxidized to obtain electrical energy.

In FIG. 1A , the system is charged: water enters inlet 1a through line 7 and is supplied to a reversible fuel cell 1 connected to a power supply 12a, and air is supplied to a reversible fuel cell 1 connected to a power supply 12a through inlet 1d. The hydrogen thus produced is supplied to reactor inlet 2a. Inside the reactor, the hydrogen reacts with the oxidized compound (I) to obtain the reduced compound (I) and a mixture H ₂ /H ₂ O. This mixture leaves the reactor 2 through outlet 2b and enters the separation unit 3 through inlet 3a. Water leaves the separation unit 3 through outlet 3c and can be recycled 5 or discharged. Hydrogen leaves the separation unit 3 through outlet 3b. The H ₂ can be recycled through line 4 or used in other ways.

In FIG. 1B , the system is discharged: water is supplied to the reactor 2 via line 7 and inlet 2a. In the reactor, water reacts with the reduced compound (I) to obtain the oxidized compound (I) and a mixture H ₂ /H ₂ O. The mixture leaves the reactor 2 via outlet 2b and enters the separation unit 3 via inlet 3a. Water leaves the separation unit 3 via outlet 3c and can be recycled or discharged via line 5. Hydrogen leaves the separation unit 3 via outlet 3b. The hydrogen is supplied to the reversible fuel cell 1 at the hydrogen inlet 1e. In the reversible fuel cell, the hydrogen reacts with the air entering from the inlet 1d to obtain water and electrical energy, which is provided at the power output 12b for further use. Unconverted hydrogen can be recycled to the separation unit 3 via line 6.

In Figure 2A, the system is charged: water enters the inlet 8a through the line 7 and is fed to the electrolyser 8 connected to the power supply 12a. The hydrogen thus produced is fed to the reactor inlet 2a. Inside the reactor, the hydrogen reacts with the oxidized compound (I) to obtain the reduced compound (I) and a mixture _H2 / _H2O . This mixture leaves the reactor 2 through the outlet 2b and enters the separation unit 3 through the inlet 3a. The water leaves the separation unit 3 through the outlet 3c and can be recycled 5 or discharged. The hydrogen leaves the separation unit 3 through the outlet 3b. This hydrogen can be recycled through the line 4 or used in other ways.

In FIG. 2B , the system is discharged: water is supplied to the reactor 2 via line 7 and inlet 2a. In the reactor, water reacts with the reduced compound (I) to obtain the oxidized compound (I) and a mixture H ₂ /H ₂ O. The mixture leaves the reactor 2 via outlet 2b and enters the separation unit 3 via inlet 3a. Water leaves the separation unit 3 via outlet 3c and can be recycled or discharged via line 5. Hydrogen leaves the separation unit 3 via outlet 3b. The hydrogen is supplied to the fuel cell 9 at the hydrogen inlet 9b. In the fuel cell, the hydrogen reacts with the air entering from inlet 9a to obtain water and electrical energy, which is provided at the power output 12b for further use. Unconverted hydrogen can be recycled to the separation unit 3 via line 6.

In Figure 3A, the system is charged: water enters inlet 8a through line 7 and is fed to electrolyzer 8 connected to power supply 12a. The hydrogen thus produced is fed to reactor inlet 2a. Inside the reactor, the hydrogen reacts with oxidized compound (I) to obtain reduced compound (I) and a mixture _H2 / _H2O . This mixture leaves reactor 2 through outlet 2b and enters first separation unit 10 through inlet 10a. Water leaves first separation unit 10 through outlet 10c and can be recycled or discharged through line 5. Hydrogen leaves first separation unit 10 through outlet 10b. This hydrogen can be recycled or used in other ways through line 4.

In FIG. 3B , the system is discharged: water is supplied to the reactor 2 via line 7 and inlet 2a. In the reactor, water reacts with the reduced compound (I) to obtain the oxidized compound (I) and a mixture H ₂ /H ₂ O. The mixture leaves the reactor 2 via outlet 2b and enters the second separation unit 11 via inlet 11a. Water leaves the second separation unit 11 via outlet 11c and can be recycled or discharged via line 5. Hydrogen leaves the second separation unit 11 via outlet 11b. The hydrogen is supplied to the fuel cell 9 at the hydrogen inlet 9b. In the fuel cell, the hydrogen reacts with the air entering from inlet 9a to obtain water and electrical energy, which is provided at the power output 12b for further use. Unconverted hydrogen can be recycled to the second separation unit 11 via line 6.

To further illustrate the present invention, the following non-limiting examples are provided.

Example 1

A single family house consuming 5200 kWh of electrical energy per year is equipped with the energy storage system of the present invention.

General equipment: For energy production, the house was equipped with 79 m2 of photovoltaic panels (peak power 13.5 kWh, estimated annual production 12549 kWh). For seasonal energy storage, an electrical energy storage system of the invention as further described below was installed (net capacity 975 kWh, estimated annual consumption 3740 kWh of electricity). For short-term energy storage, rechargeable batteries (48 kWh Flex Storage P, VARTA Storage GmbH) were installed.

Energy storage system: The energy storage reactor has a cylindrical shape, an inner diameter of 1.35 m, and an inner length of 1.35 m. The reactor vessel consists of stainless steel (1.4404/316L), and the flat sealing ring consists of expanded graphite (Novaphit SSTC, Angst+Pfister AG). The reactor is insulated using rock wool (Flumroc FMI-500FP Alum, Indisol AG). The reactor vessel is charged with 24.8 kmol FeOx in the form of goethite (Bayferrox 3920, Lanxess AG).

The gases leaving the reactor were cooled in a tube bundle heat exchanger (M&C TechGroup Germany GmbH). Condensed water left the heat exchanger via a condensate drain (M&C TechGroup Germany GmbH). The gases leaving the heat exchanger passed through a back pressure regulator (Swagelock) that maintained the pressure at the heat exchanger outlet at 1.05 bar (absolute). The return of the gases through the pressure to the reactor vessel was facilitated by a pump (N186.1.2ST.9E Ex, KNF Neuberger AG).

For safety reasons, the reactor was equipped with four low-pressure regulators: two to protect against overpressure (1.5 bar (absolute pressure), type ZM-B, Zimmerli Messtechnik AG) and two to protect against underpressure (0.8 bar (absolute pressure), type ZM-R, Zimmerli Messtechnik AG). The underpressure regulators were connected to the pressurized nitrogen source and the overpressure regulators were connected to the safety outlet.

The fixed bed is heated by multiple internal tubular heating elements (RPT, K. The fixed bed temperature was measured at multiple locations using J-type thermocouples. In addition, the reactor jacket and lid temperatures were measured at multiple locations using K-type surface thermocouples.

Charging: 4572 kWh of surplus photovoltaic energy was produced between March and October. This energy was converted into 52.9 kg of hydrogen in two electrolysers (QL2000, Fuel Cell Store, assuming 45% efficiency relative to H2-HHV) and used to charge the energy storage reactor.

Discharge: Between November and February, an energy shortage of 962 kWh was compensated by discharging 52.5 kg of hydrogen from the energy storage reactor and converting it in two fuel cells (Horizon H-500PEM fuel cells, Fuel Cell Shop, assumed efficiency of 55% relative to H2-LHV).

Conclusion: The system operates safely and reduces the amount of external energy required.

Embodiment 2:

A residential building with an electrical energy consumption of approximately 10400 kWh/year was equipped with the energy storage system of the invention.

General equipment: For energy production, the house is equipped with photovoltaic panels of about 150 square meters (about 25 kW peak power, estimated annual production of about 25500 kWh). For seasonal energy storage, an electrical energy storage system according to the invention as further described below is installed. It has a net electrical energy storage capacity of about 1800 kWh and consumes about 16900 kWh of electrical energy per year (including the electrolyzer). For short-term energy storage, rechargeable batteries are installed.

Energy storage system: The energy storage reactor has a cylindrical shape, a diameter of about 1.6 m, and a length of about 1.8 m. The reactor vessel consists of stainless steel (1.4571/316Ti). The flat sealing ring consists of expanded graphite. The reactor is insulated using rock wool. The reactor vessel is filled with iron oxide containing a total of about 52 kmol Fe. Rock wool ( Flumroc AG) is deposited in compressed form on top of the iron oxide and thus takes over the function of the compensating element.

Heat exchangers are used to transfer heat from the fluid leaving the reactor to the fluid entering the reactor.

A gas cooler is used to further cool the cooling fluid leaving the heat exchanger. Condensate leaves the heat exchanger through a condensate drain.

During charging, the gas leaving the gas cooler is facilitated by a gas pump back to the reactor vessel. During discharging, the gas leaving the gas cooler is fed to the fuel cell.

For safety reasons, the reactor is equipped with four safety valves: two to prevent overpressure and two to prevent underpressure. The underpressure regulator is connected to the pressurized nitrogen source and the overpressure regulator is connected to the safety outlet.

The reactor vessel was heated externally by multiple heating bands, and the surface temperature of the reactor was measured at multiple locations.

Charging: Between March and October, about 8500 kWh of surplus photovoltaic energy was produced. This energy was converted into about 110 kg of hydrogen in two electrolysers (with an efficiency of about 50% relative to H2-HHV) for charging the energy storage reactor.

Discharge: Between November and February, an energy shortage of about 1800 kWh was compensated by discharging about 110 kg of hydrogen from the energy storage reactor and converting it in two fuel cells (efficiency about 50% relative to H2-LHV).

Thermal integration: The energy storage system is located in the basement of a residential building. Between November and February, the system provides approximately 2000 kWh of heat at approximately 25°C to the residential heating system and the fuel cell provides approximately 1800 kWh of heat recovered in water at approximately 60°C.

Conclusion: The system operates safely and reduces the amount of external energy required. The electricity produced by the photovoltaic panels is stored between March and October. The electricity and heat are released between November and February. Heat integration reduces the amount of energy required to heat the residential building and produce warm water during the discharge period.

Embodiment 3:

A. Experimental Setup

An energy storage system according to the invention was built and operated ("pilot"). The system contained a thermally insulated energy storage reactor according to the invention, hydrogen and nitrogen supplies, a water supply, a gas cooler equipped with a condensate drain, a gas purge, a gas recirculation system and process control software.

Reactor: The cylindrical stainless steel (1.4404/316L) reactor vessel had an inner diameter of 398 mm, an inner height of 1200 mm, and an internal volume of approximately 150 L. The flat top and bottom covers (10 mm thick) of the reactor were connected to the jacket (4 mm thick) via flange connections equipped with flat-pressure sealing rings (NOVAPHIT SSTC/PASSO3, 1.5 mm thick, Angst+Pfister AG).

For safety reasons, the reactor was equipped with four low-pressure regulators: two to prevent overpressure (+500 mbar, type ZM-B, Zimmerli Messtechnik AG) and two to prevent underpressure (-200 mbar, type ZM-R, Zimmerli Messtechnik AG). The inlet of the underpressure regulator was connected to the pressurized nitrogen source, while the inlet of the overpressure regulator was connected to the safety outlet.

The reactor was equipped with a gas inlet at the top, a water inlet at the top, and an outlet at the bottom.

The reactor interior was heated as follows: the reactor top cover was equipped with 8 stainless steel tubes (outer diameter 6.35 mm, wall thickness 0.89 mm). These steel tubes (about 1 m long) extended into the reactor and were sealed at their ends. A tubular heating element (RPT 4X 1150 mm 630 W 230 V, heating length 1000 mm, Fe-CuNi thermocouple, K. AG). Four separate heating sections were constructed, each consisting of two heating elements connected in series. The maximum output power of the heating sections was therefore 315W, and the maximum power of the entire heating system was 1260W.

The temperature inside the reactor was measured at multiple locations using 8 J-type thermocouples. The temperature of the reactor jacket, top cover, and bottom cover was measured at 14 locations using K-type thermocouples.

The reactor was thermally insulated by covering all sides of the reactor with approximately 15-25 cm of rock wool (Flumroc FMI-500FP Alu, Indsol AG).

Hydrogen supply and nitrogen supply: Hydrogen (5.0, Pangas) was supplied to a hydrogen mass flow controller (EL-FLOW Select, Bronkhorst AG) at about 1.5 bar (absolute pressure). Nitrogen (5.0, Pangas) was supplied to a nitrogen mass flow controller (5850EM, Brooks Instrument) at about 1.5 bar (absolute pressure). The outlets of the two mass flow controllers were combined in the first tee and supplied to a forward pressure reducing valve (Swagelok) whose pressure was set to 1.0-1.3 bar (absolute pressure). The gas passing through the pressure reducing valve was combined with the recycled gas flow in the second tee. The gas flow was piped to the gas inlet of the reactor.

Water supply: Deionized water (ETH Zurich) was pumped (ISMATEC REGLO-Z Digital, Cole-Parmer GmbH) through heated (400 W, ETH Zurich) steel pipes. The water leaving the evaporator was piped to the water inlet of the reactor.

Gas cooler with condensate drain: The outlet of the reactor is connected to a water-cooled tube bundle heat exchanger (LGT 2, M&C TechGroup Germany GmbH) equipped with automatic condensate drain (ADS-SS, M&C TechGroup Germany GmbH). The condensate is collected in a bottle. The cooled gas is piped to the third tee via a hydrogen mass flow meter (EL-Flow Select, Bronkhorst AG). The mass flow meter is used to measure the flow of the gas to be purified or recycled.

Purification: The first outlet of the third tee is equipped with a needle valve, which can be opened to release the gas to the safety outlet.

Gas recirculation: The second outlet of the third tee was part of the gas recirculation, which further consisted of a needle valve, a solenoid valve (Type 6240, Bürkert Schweiz AG), a gas pump (N 026.1.2ST.9EEx, KNF Neuberger AG) and the inlet of the second tee. The pressure in the system was monitored at the reactor top and at the gas outlet of the gas cooler using a digital pressure gauge (Keller AG).

B. Operation

The reactor was filled to the top with 75 kg of FeOx in the form of goethite (Bayferrox 3920, Lanxess AG). Cooling water was always supplied to the gas cooler (50 L/h, water temperature 10-12° C.). Taking into account the internal heating system, only the core of the fixed bed (65% of the inner diameter, 88% of the inner height) was hot enough to be converted into iron, while the remaining goethite was only reduced to magnetite and served as thermal insulation in the reactor.

First charge: Initially, the atmosphere in the reactor was changed from air to nitrogen, and the set temperature of the heating element was increased to 200°C over 5 days. The atmosphere in the reactor was then changed from nitrogen to hydrogen. In the following, the temperature changes were as follows:

First discharge: The first discharge started after 92.7 days and ended after 102.9 days. During this period, the following 11 runs were performed, which differed in duration, set temperature (of the reactor), set temperature of the evaporator and setting of the water pump.

run Starting time duration Reactor set temperature Evaporator set temperature Pump settings # [Days] [h] [℃] [℃] [-] 1 92.75 5.37 380 200 5 2 95.71 7.89 310 200 3 3 96.04 0.22 310 120 3 4 96.05 19.29 305 120 3 5 96.86 4.27 305 150 5 6 97.04 6.38 265 150 3 7 98.73 2.35 300 150 5 8 98.83 2.38 300 150 7 9 98.93 1.37 300 200 10 10 98.99 0.33 300 200 3 11 99.02 15.62 300 200 3

Second charge: After 102.9 days, the second charge program started at a set reactor temperature of 200°C. The temperature was increased to 380°C with a heating ramp. The program ended after 117.8 days at a set temperature of 320°C:

time Reactor set temperature time Reactor set temperature [Days] [℃] [Days] [℃] 102.94 200 117.23 380 103.01 200 117.23 320 103.04 200 117.82 320 103.16 380

Second discharge: The second discharge started after 117.8 days and ended after 120.0 days. During this period, the following 7 runs were performed, with different durations, set temperatures (of the reactor) and water pump settings:

run Starting time duration Reactor set temperature Evaporator set temperature Pump settings # [Days] [h] [℃] [℃] [-] 1 117.84 0.19 320 200 3 2 117.84 0.14 320 200 5 3 117.85 0.38 320 200 9 4 117.87 7.99 320 200 12 5 118.62 9.33 280 200 10 6 119.01 24.70 280 200 8 7 120.04 13.19 280 200 7

C. Results:

The state of charge of the energy storage system is determined by the water balance. Specifically, the mass of water collected at the condensate drain minus the mass of water supplied to the evaporator minus the mass of water generated when goethite is initially converted to magnetite (10.08 kg) to obtain the net water amount. This net water amount is proportional to the state of charge. Therefore, 0% charge corresponds to 0 kg of net water, and 100% charge corresponds to about 7.5 kg of net water, i.e., the hot core is completely converted to iron. The table below shows the net water amount and state of charge after charge/discharge.

Clean water [kg] charging Before first charging -10.1 After the first charge 4.14 54.8% After the first discharge 0.26 3.4% After the second charge 5.88 78.0% After the second discharge 0.36 4.7% After the third charge 5.98 79.2% After the third discharge 0.35 4.7%

D. Conclusion:

A pilot test of the energy storage system was built and safely operated at low pressure. In the second and third cycles, the net amount of water changed by about 5.5 kg, which corresponds to about 600 g of hydrogen.

The above data also show the stability of the system in terms of charging/discharging.

Embodiment 1. A reactor (2), comprising:

■ a reaction vessel (2c) with a limited internal volume,

■Compensation element (2d),

■Inlet and outlet (2a, 2b),

■Heating element (2e),

■Insulator (2f),

Features:

■The internal volume is filled with a fixed bed;

■ the fixed bed contains or consists of particles of formula (I), i.e., FeOx(I), wherein 0≤x≤1.5; and

■ the fixed bed is free of cavities, the cavities being characterized in that they have a volume greater than 10000 ^mm3 and all the main radii are at least 3 mm long, the main radii being the three main radii of the largest ellipsoid that fits into the cavity;

■ The compensating element is suitable for adjusting the inner volume.

Embodiment 2. The reactor according to embodiment 1, wherein the reaction vessel (2c)

■ limited internal volume of 1–10m ³ ; and/or

■ in the form of a cylinder or a sphere or a bundle of tubes, preferably in the form of a cylinder; and/or

■No tools for stirring or transport are included.

Embodiment 3. The reactor according to embodiment 1 or 2, wherein the compensating element (2d)

■ suitable for compensating for volume expansion/compression during operation of the reactor; and/or

■ Configured in the form of a membrane or in the form of a piston or in the form of a sponge or in the form of a fibrous material.

Embodiment 4. A reactor according to any one of embodiments 1-3, wherein the inlet and outlet (2a, 2b)

■ suitable for filling the inner volume and removing gas from the inner volume;

■ is suitable for retaining particulate material within the reactor; and/or

■ Located at the top (inlet) and bottom (outlet) of the reactor (2).

Embodiment 5. The reactor according to any one of embodiments 1-4, further comprising one or more of the following elements:

A gas tank (containing an inert gas) is in fluid communication with the reaction vessel to regulate gas pressure and volume during operation.

Embodiment 6. An energy storage system (13), comprising

■A reactor (2) according to any one of embodiments 1 to 5, wherein the reactor is in communication with the following fluid:

■ Units (1), (8, 9) operating as reversible fuel cells,

■ Unit for separating water from hydrogen (3).

Embodiment 7. A system according to Embodiment 6, wherein the reversible fuel cell is replaced by an electrolyzer (8) and a fuel cell (9), wherein the electrolyzer (8) is fluidically connected to the reactor (2), and the fuel cell (9) is fluidically connected to the separation unit (3).

Embodiment 8. A system according to embodiment 6 or 7, wherein the separation unit (3) is replaced by two separation units (10, 11) for charging mode and discharging mode.

Embodiment 9. The system of any one of Embodiments 6-8 further comprises a chamber containing a microporous hygroscopic material within the reactor.

Embodiment 10. A method for storing hydrogen, comprising the following steps: reducing a compound of formula (I) in the reactor by supplying a gas containing hydrogen to the reactor described in any one of Embodiments 1-5, thereby obtaining a reduced compound of formula (I) and water.

Embodiment 11. A method for generating hydrogen, comprising the following steps: oxidizing a compound of formula (I) in a reactor by supplying water to the reactor described in any one of Embodiments 1-5, thereby obtaining an oxidized compound of formula (I) and hydrogen.

Embodiment 12. A method for storing electrical energy, the method comprising:

(a) electrolyzing and reducing water to obtain hydrogen;

(b) reducing the compound of formula (I) in the reactor according to any one of embodiments 1 to 5 by supplying a gas comprising hydrogen to the reactor, thereby obtaining a H ₂ O/H ₂ gaseous mixture;

(c) separating _H2 from the gaseous mixture and recycling the _H2 to the reactor.

Embodiment 13. A method of releasing electrical energy, the method comprising:

(d) oxidizing the compound of formula (I) in the reactor of any one of embodiments 1 to 5 by feeding water to the reactor, thereby obtaining an oxidized compound of (I) and a _H2O / _H2 gaseous mixture,

(e) separating _H2 from the gaseous mixture, and

(f) Electrochemical reduction of _H2 to obtain water and electrical energy.

Embodiment 14. Use of the reactor according to any one of embodiments 1-5 for:

■ converting hydrogen into water by reducing the compound of formula (I) and storing the energy obtained thereby; and/or

■ The conversion of water into hydrogen by oxidation of the compound of formula (I) releases previously stored energy.

Embodiment 15. Use of the system according to any one of Embodiments 6-9 for storing electrical energy to charge the system; and/or for releasing electrical energy to discharge the system.

Claims (17)

1. A reactor, comprising:

■ A reaction vessel defining an interior volume and,

■ The compensation element is arranged to be connected to the first and second compensation elements,

■ An inlet and an outlet,

■ The heating element is arranged such that,

■ The insulator is provided with a plurality of insulating layers,

The method is characterized in that:

■ The inner volume is filled with a fixed bed;

■ The fixed bed contains or consists of particles of the formula (I), namely FeOx (I), wherein x is more than or equal to 0 and less than or equal to 1.5; and is also provided with

■ The fixed beds are free of cavities characterized in that they have a volume greater than 10000mm ³ and all major radii are at least 3mm long, the major radii being the three major radii of the largest ellipsoid that fits the cavity;

■ The compensation element is adapted to adjust the inner volume.

2. The reactor of claim 1, wherein the reaction vessel

■ Defining an internal volume of 1-10m ³; and/or

■ In the form of a cylinder or sphere or tube bundle; and/or

■ No means for agitation or transport are involved.

3. The reactor of claim 2, wherein the reaction vessel is in the form of a cylinder.

4. A reactor according to any one of claims 1-3, wherein the compensating element

■ Is adapted to compensate for volume expansion/compression during operation of the reactor; and/or

■ Configured in the form of a membrane or in the form of a piston or in the form of a sponge or in the form of a fibrous material.

5. A reactor according to any one of claims 1 to 3, wherein the inlet and outlet ports

■ Is adapted to fill the inner volume and remove gas from the inner volume;

■ Adapted to retain particulate material within the reactor; and/or

■ At the top and bottom of the reactor, wherein the inlet is at the top of the reactor and the outlet is at the bottom of the reactor.

6. A reactor according to any one of claims 1-3, further comprising one or more of the following elements:

A gas tank in fluid communication with the reaction vessel to regulate gas pressure and volume during operation.

7. The reactor of claim 6, wherein the gas tank contains an inert gas.

8. An energy storage system comprising

■ The reactor of any one of claims 1-7, in fluid communication with:

■ As a unit for operation of the reversible fuel cell,

■ A separation unit for separating water from hydrogen.

9. The system of claim 8, wherein the reversible fuel cell is replaced with an electrolyzer in fluid communication with the reactor and a fuel cell in fluid communication with the separation unit.

10. The system according to claim 8 or 9, wherein the separation unit is replaced by two separation units for a charging mode and a discharging mode.

11. The system of claim 8 or 9, further comprising a chamber within the reactor containing a microporous hygroscopic material.

12. A method of storing hydrogen comprising the steps of: reducing the compound of formula (I) in the reactor by supplying a hydrogen-containing gas to the reactor of any one of claims 1-7, thereby obtaining a reduced compound of formula (I) and water.

13. A method of generating hydrogen comprising the steps of: oxidizing a compound of formula (I) in a reactor according to any one of claims 1-7 by feeding water to said reactor, thereby obtaining an oxidized compound of formula (I) and hydrogen.

14. A method of storing electrical energy, the method comprising:

(a) Electrolytically reducing the water to obtain hydrogen;

(b) Reducing a compound of formula (I) in a reactor according to any one of claims 1-7 by feeding a hydrogen-containing gas to said reactor, thereby obtaining a H ₂O/H₂ gaseous mixture;

(c) H ₂ is separated from the gaseous mixture and the H ₂ is recycled to the reactor.

15. A method of releasing electrical energy, the method comprising:

(d) Oxidizing a compound of formula (I) in a reactor according to any one of claims 1-7 by feeding water to said reactor, thereby obtaining an oxidized compound of formula (I) and a gaseous mixture of H ₂O/H₂,

(E) Separating H ₂ from the gaseous mixture, and

(F) H ₂ is electrochemically reduced to obtain water and electrical energy.

16. Use of a reactor according to any one of claims 1-7 for:

■ Converting hydrogen into water by reduction of the compound of formula (I) and storing the energy thus obtained; and/or

■ The previously stored energy is released by oxidizing the compound of formula (I) to convert water to hydrogen.

17. Use of a system according to any of claims 8-11 for storing electrical energy to charge the system; and/or for releasing electrical energy to discharge the system.